Technical Field
The present disclosure relates to a mirror package, in particular for a picoprojector, comprising micromirrors obtained using the MEMS technology.
Description of the Related Art
As is known, picoprojectors are devices of small dimensions which are able to project, on a wall or other surface, images, video clips, and other visual documents stored in small portable electronic equipment, such as cellphones, tablets, laptops, and the like. Furthermore, it has already been proposed to integrate such picoprojectors on the dashboard of motor vehicles, for example for projecting onto the windscreen information on satellite-navigation functions and other route or communication information.
In order to reduce the size, picoprojectors use MEMS micromirror systems, which are able to turn about two axes in order to perform a movement of scanning of a two-dimensional area. In particular, in some solutions, micromirror systems comprise a pair of MEMS mirrors driven for turning about two mutually perpendicular rotation axes.
For instance, FIG. 1 is a schematic illustration of a picoprojector wherein a light source 1, typically a laser source, generates a light beam 2 (generally formed by three monochromatic beams, one for each base color), which, through an optical system 3 shown only schematically, is deflected by a pair of MEMS mirrors 5, 6. The first MEMS mirror 5 may, for example, be a horizontal micromirror, rotating about a first axis A and generating a fast horizontal scan, and the second MEMS mirror 6 may, for example, be a vertical micromirror, rotating about a second axis B, transverse, in particular perpendicular, to the first axis A, and generating a slow vertical scan, typically of a sawtooth type. The combination of the movements of the two MEMS mirrors 5, 6 causes the light beam 2 to perform a complete two-dimensional scanning movement and, once projected on a projection display 7, to generate a two-dimensional image thereon. Such a system is described, for example, in WO 2010/067354, which also published as U.S. Pat. Pub. No. 2011/234898.
MEMS mirrors 5, 6 may be formed as shown in FIG. 2. Here, a chip 10 comprises a mirror element 11 having a reflecting surface 9 and suspended over a semiconductor substrate 8. The mirror element 11 is supported by a pair of arms 12, which extend from opposite sides of the mirror element 11 and define the rotation axis of the mirror element 11 (for example, rotation axis B of the vertical micromirror 6). The arms 12 are connected to a fixed peripheral portion 13 of the chip 10, which is fixed with respect to the substrate 8, via torsion springs 14 enabling rotation of the arms 12 about the axis B. The arms 12 are further coupled to a driving structure formed by actuation units 18 of an electrostatic type. Each actuation unit 18 comprises mobile electrodes 19 facing fixed electrodes 20.
In detail, the mobile electrodes 19 are fixed with respect to the arms 12 and are comb-fingered with respect to the fixed electrodes 20 for generating a capacitive coupling. The fixed electrodes 20 are carried by supporting regions 21, which are generally fixed with respect to the substrate 8 of the chip 10. By virtue of the arrangement of the electrodes 19, 20, the driving structure is also defined as “comb-drive structure”.
By applying appropriate voltages between the mobile electrodes 19 and the fixed electrodes 20, it is possible to generate attraction/repulsion forces between them and cause rotation of the mobile electrodes 19 with respect to the fixed electrodes 20, rotation of the arms 12 about axis B, and thus the corresponding controlled rotation of the mirror element 11.
Currently, MEMS mirrors 5, 6 are manually mounted in a picoprojector, each already fixed to a respective support. For instance, FIG. 3 shows a mirror package 24 wherein a chip, designated again by 10, is fixedly coupled to a support 25, generally of metal. An electric connection structure 26, for example a flexible printed circuit (FPC) carrying electric connection lines 27, is also fixedly coupled to the support 25. The electric connection lines 27 are connected to the chip 10 via electric wires 28, in a per se known manner.
During assembly in a portable electronic apparatus, an operator picks up two mirror chips, one for a horizontal MEMS mirror and the other for a vertical MEMS mirror, and using a jig and microactuators in a specific apparatus, positions the two chips to mate them, until they are brought into the desired alignment. Next, the operator applies a curable glue using ultraviolet light, and the two mirror chips are fixed in the operative position.
This type of assembly process is slow, difficult, and sensibly error-prone. It follows that, with this method, the throughput and yield of correctly mounted pieces are not optimal.